A hydraulic power steering system includes a pump (1), a flow control valve (4), and a bypass control valve (20). The flow control valve (4) responds to a pressure drop across a metering orifice (3) disposed in a fluid supply passage (2) so as to control the flow rate of operational fluid by bypassing part of the operational fluid to a reservoir (11). A spring chamber (6a) of the flow control valve (4) is connected to the supply passage (2) through a control orifice (7). The bypass control valve (20) is disposed between the spring chamber (6a) and the reservoir (11) so as to increase the amount of bypassed fluid by controlling the pressure in the spring chamber (6a). The bypass control valve (20) has a control spool (21), a load pressure introduction port (23A) connected to the upstream side of the control orifice (7) for leading the load pressure to a first end of the control spool (21) and a pilot port (23B) connected to the downstream side of the control orifice (7) for leading the pressure in the spring chamber (6a) of the flow control valve (4) to a second end of the control spool (21). Moreover, a pressure receiving area of the second end of the control spool (21) is smaller than that of the first end of the control spool (21). Accordingly, a spring (22) having a small spring constant can be used in the bypass control valve (20), even when the control spool (21) is controlled within a short stroke.